It is known that 240 -deoxy-5-fluorouridine (FdUrd) significantly inhibits the growth of tumor cells in in vitro tests. But when tested in vivo, FdUrd can not produce a satisfactory therapeutic effect under the influence of pyrimidine nucleoside phosphorylase acting as a decomposition enzyme. For this reason, FdUrd has not been widely used as a anti-cancer agent. Yet, FdUrd has been used for curing specific organic cancers although solely by a continuous administration method (Cancer 57 492-498 (1986), J. Urol. 139 259-262 (1988)). This suggests the possibility that FdUrd might be used as a clinically potent anti-cancer agent if the effective concentration of FdUrd in blood can be retained for a prolonged period. The production of various FdUrd derivatives has been attempted to improve said property (Japanese Unexamined Patent Publications Nos. 113797/1981, 109722/1982, 99499/1983 and 238797/1986). The compounds disclosed in the publications enable effective concentration of FdUrd in blood for a long period. But it was found that the compounds cause side effects such as diarrhea because of the increase of FdUrd concentration in blood beyond its effective concentration in blood, failing to produce a satisfactory clinical effect.
More recently, FdUrd derivatives were reportedly prepared with particular attention directed to the ability of FdUrd to sustain its low concentration in blood (Japanese Unexamined Patent Publications Nos. 106593/1986, 104093/1989 and 199992/1989). The compounds disclosed in the publications achieve an improvement in continuously releasing FdUrd into blood, but sustain the effective concentration of FdUrd in blood to a lesser extent than a continuous administration method because of a low conversion ratio from the compound to FdUrd, failing to give the desired therapeutic index.